RADIAL EVOLUTION OF THE PROTON BEAM DENSITY IN THE SOLAR WIND

In the solar wind the proton velocity distributions usually have two maxima. The part of the distribution around the major maximum of the distribution is called the core part, and that around the second maximum is called the beam part. How the proton beam is formed in the solar wind is still a problem to be solved. The radial evolution of the beam density places an observational constraint on the theory. To clarify the issue the plasma data obtained by Helios 2 in high-speed wind with 700 km/s > v_s > 600 km/s was analyzed. And the 3-D proton velocity distributions were integrated to get a 1-D distribution along the magnetic field direction. The mean values and standard deviations of the ratios of the beam density relative to the total proton density in 0.05 AU interval were calculated. It was found that the relative beam density in high-speed wind doesn't increase obviously with increasing heliocentric distance, thus it is concluded that the proton beam probably is formed inside 0.3 AU, and doesn't change much during the solar wind expanding outward.